1. Field of the Invention
The present invention relates to antennas and more particularly to a low profile antenna which is small in size, simple in construction and high in efficiency.
2. Description of the Prior Art
In the past, many portable radio devices and associated equipment have employed external whip antennas for transmitting or receiving purposes. Unfortunately, such antennas when mounted normal to a surface tend to significantly increase overall dimensions of the portable radio device of which it is a part, which may be a prohibiting factor especially when compactness is a primary consideration. Furthermore, the external whip antennas usually extend out from the radio device in an awkward, cumbersome manner, thus causing a substantial increase in the overall longitudinal dimensions of the radio device.
In an effort to reduce the overall height of vertical antennas, such antennas are often compressed into helical antennas. Unfortunately, although such helical antennas exhibit a reduced overall vertical dimension, they are not as efficient as their full size vertical counterparts.
Further, the copending application entitled Two Element Low Profile Antenna, Ser. No. 489,894 having a filing date of Apr. 29th, 1983 discloses an antenna configuration which provides a more compact low profile antenna. The low profile antenna disclosed therein comprises a counterpoise of electrically conductive material and a passive element oriented substantially parallel thereto. The ends of the passive element are electrically coupled to the counterpoise surface. The active element is made of electrically conductive material and includes a middle portion and first and second outer end portions. The middle portion is situated adjacent and spaced apart from the passive element by a predetermined distance and in a parallel relationship therewith. The first outer end portion of the outer element is bent toward the grounded end of the passive element nearest thereto. The first outer end portion represents the feed point of the antenna with respect to the counterpoise. The remaining second outer end portion of the active element is bent towards the remaining end of the passive element nearest thereto and is electrically coupled to the counterpoise surface. The first and second outer portions by virtue of the bends which orient them close to the ends of the passive element result in coupling substantial electromagnetic energy between the active element and the passive element. For this antenna the critical coupling required for impedance matching occurs at the low impedance points which are at both ends of the antenna in the sections perpendicular to the counterpoise, the critical coupling being induced by the magnetic field. Further, both of the antenna elements are approximately one-half wave length long at the selected operating frequency.